Microlens structures for semiconductor device with single-photon avalanche diode pixels

ABSTRACT

An imaging device may include a plurality of single-photon avalanche diode (SPAD) pixels. The SPAD pixels may be overlapped by microlenses to direct light incident on the pixels onto photosensitive regions of the pixels and a containment grid with openings that surround each of the microlenses. During formation of the microlenses, the containment grid may prevent microlens material for adjacent SPAD pixels from merging. To ensure separation between the microlenses, the containment grid may be formed from material phobic to microlens material, or phobic material may be added over the containment grid material. Additionally, the containment grid may be formed from material that can absorb stray or off-angle light so that it does not reach the associated SPAD pixel, thereby reducing crosstalk during operation of the SPAD pixels.

BACKGROUND

This relates generally to imaging systems and, more particularly, toimaging systems that include single-photon avalanche diodes (SPADs) forsingle photon detection.

Modern electronic devices such as cellular telephones, cameras, andcomputers often use digital image sensors. Image sensors (sometimesreferred to as imagers) may be formed from a two-dimensional array ofimage sensing pixels. Each pixel typically includes a photosensitiveelement (such as a photodiode) that receives incident photons (light)and converts the photons into electrical signals. Each pixel may alsoinclude a microlens that overlaps and focuses light onto thephotosensitive element. Image sensors are sometimes designed to provideimages to electronic devices using a Joint Photographic Experts Group(JPEG) format.

Conventional image sensors with backside-illuminated pixels may sufferfrom limited functionality in a variety of ways. For example, someconventional image sensors may not be able to determine the distancefrom the image sensor to the objects that are being imaged. Conventionalimage sensors may also have lower than desired image quality andresolution.

To improve sensitivity to incident light, single-photon avalanche diodes(SPADs) may sometimes be used in imaging systems. However, SPADs mayrequire larger photosensitive regions than conventional image sensorsand therefore may require thicker microlenses to focus light on thephotosensitive elements within the SPADs. In order to apply microlensesthick enough to focus light in a desired manner, high viscosity materialmay be required. It may be difficult to control uniformity, patterning,and reflow characteristics when using high viscosity materials.

It would therefore be desirable to be able to provide improved microlensstructures for single-photon avalanche diode pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an illustrative single-photonavalanche diode pixel in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative silicon photomultiplier inaccordance with an embodiment.

FIG. 3 is a schematic diagram of an illustrative imaging system with aSPAD-based semiconductor device in accordance with an embodiment.

FIG. 4 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals in a SPAD-basedsemiconductor device in accordance with an embodiment.

FIG. 5 is a cross-sectional diagram of illustrative SPAD pixels coveredwith microlenses in accordance with an embodiment.

FIG. 6 is a cross-sectional diagram of illustrative SPAD pixels coveredwith microlenses that are separated by a containment grid in accordancewith an embodiment.

FIG. 7 is a process flow diagram of an illustrative method of forming acontainment grid and interspersed microlenses in accordance with anembodiment.

FIG. 8 is a process flow diagram of an illustrative method of forming acontainment grid and interspersed microlenses using phobic material overthe containment grid in accordance with an embodiment.

FIG. 9 is a process flow diagram of an illustrative method of forming acontainment grid and interspersed microlenses using phobic material thatsurrounds portions of the containment grid in accordance with anembodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to imaging systems thatinclude single-photon avalanche diodes (SPADs).

Some imaging systems include image sensors that sense light byconverting impinging photons into electrons or holes that are integrated(collected) in pixel photodiodes within the sensor array. Aftercompletion of an integration cycle, collected charge is converted into avoltage, which is supplied to the output terminals of the sensor. Incomplementary metal-oxide semiconductor (CMOS) image sensors, the chargeto voltage conversion is accomplished directly in the pixels themselvesand the analog pixel voltage is transferred to the output terminalsthrough various pixel addressing and scanning schemes. The analog pixelvoltage can also be later converted on-chip to a digital equivalent andprocessed in various ways in the digital domain.

In single-photon avalanche diode (SPAD) devices (such as the onesdescribed in connection with FIGS. 1-4), on the other hand, the photondetection principle is different. The light sensing diode is biasedslightly above its breakdown point and when an incident photon generatesan electron or hole, this carrier initiates an avalanche breakdown withadditional carriers being generated. The avalanche multiplication mayproduce a current signal that can be easily detected by readoutcircuitry associated with the SPAD. The avalanche process needs to bestopped (quenched) by lowering the diode bias below its breakdown point.Each SPAD may therefore include a passive and/or active quenchingcircuit for quenching the avalanche.

This concept can be used in two ways. First, the arriving photons maysimply be counted (e.g., in low light level applications). Second, theSPAD pixels may be used to measure photon time-of-flight (ToF) from asynchronized light source to a scene object point and back to thesensor, which can be used to obtain a 3-dimensional image of the scene.

FIG. 1 is a circuit diagram of an illustrative SPAD device 202. As shownin FIG. 1, SPAD device 202 includes a SPAD 204 that is coupled in serieswith quenching circuitry 206 between a first supply voltage terminal 208(e.g., a ground power supply voltage terminal) and a second supplyvoltage terminal 210 (e.g., a positive power supply voltage terminal).During operation of SPAD device 202, supply voltage terminals 208 and210 may be used to bias SPAD 204 to a voltage that is higher than thebreakdown voltage. Breakdown voltage is the largest reverse voltage thatcan be applied without causing an exponential increase in the leakagecurrent in the diode. When SPAD 204 is biased above the breakdownvoltage in this manner, absorption of a single-photon can trigger ashort-duration but relatively large avalanche current through impactionization.

Quenching circuitry 206 (sometimes referred to as quenching element 206)may be used to lower the bias voltage of SPAD 204 below the level of thebreakdown voltage. Lowering the bias voltage of SPAD 204 below thebreakdown voltage stops the avalanche process and correspondingavalanche current. There are numerous ways to form quenching circuitry206. Quenching circuitry 206 may be passive quenching circuitry oractive quenching circuitry. Passive quenching circuitry may, withoutexternal control or monitoring, automatically quench the avalanchecurrent once initiated. For example, FIG. 1 shows an example where aresistor is used to form quenching circuitry 206. This is an example ofpassive quenching circuitry. After the avalanche is initiated, theresulting current rapidly discharges the capacity of the device,lowering the voltage at the SPAD to near to the breakdown voltage. Theresistance associated with the resistor in quenching circuitry 206 mayresult in the final current being lower than required to sustain itself.The SPAD may then be reset to above the breakdown voltage to enabledetection of another photon.

This example of passive quenching circuitry is merely illustrative.Active quenching circuitry may also be used in SPAD device 202. Activequenching circuitry may reduce the time it takes for SPAD device 202 tobe reset. This may allow SPAD device 202 to detect incident light at afaster rate than when passive quenching circuitry is used, improving thedynamic range of the SPAD device. Active quenching circuitry maymodulate the SPAD quench resistance. For example, before a photon isdetected, quench resistance is set high and then once a photon isdetected and the avalanche is quenched, quench resistance is minimizedto reduce recovery time.

SPAD device 202 may also include readout circuitry 212. There arenumerous ways to form readout circuitry 212 to obtain information fromSPAD device 202. Readout circuitry 212 may include a pulse countingcircuit that counts arriving photons. Alternatively or in addition,readout circuitry 212 may include time-of-flight circuitry that is usedto measure photon time-of-flight (ToF). The photon time-of-flightinformation may be used to perform depth sensing.

In one example, photons may be counted by an analog counter to form thelight intensity signal as a corresponding pixel voltage. The ToF signalmay be obtained by also converting the time of photon flight to avoltage. The example of an analog pulse counting circuit being includedin readout circuitry 212 is merely illustrative. If desired, readoutcircuitry 212 may include digital pulse counting circuits. Readoutcircuitry 212 may also include amplification circuitry if desired.

The example in FIG. 1 of readout circuitry 212 being coupled to a nodebetween diode 204 and quenching circuitry 206 is merely illustrative.Readout circuitry 212 may be coupled to any desired portion of the SPADdevice. In some cases, quenching circuitry 206 may be consideredintegral with readout circuitry 212.

Because SPAD devices can detect a single incident photon, the SPADdevices are effective at imaging scenes with low light levels. Each SPADmay detect how many photons are received within a given period of time(e.g., using readout circuitry that includes a counting circuit).However, as discussed above, each time a photon is received and anavalanche current initiated, the SPAD device must be quenched and resetbefore being ready to detect another photon. As incident light levelsincrease, the reset time becomes limiting to the dynamic range of theSPAD device (e.g., once incident light levels exceed a given level, theSPAD device is triggered immediately upon being reset).

Multiple SPAD devices may be grouped together to increase dynamic range.FIG. 2 is a circuit diagram of an illustrative group 220 of SPAD devices202. The group of SPAD devices may be referred to as a siliconphotomultiplier (SiPM). As shown in FIG. 2, silicon photomultiplier 220may include multiple SPAD devices that are coupled in parallel betweenfirst supply voltage terminal 208 and second supply voltage terminal210. FIG. 2 shows N SPAD devices 202 coupled in parallel (e.g., SPADdevice 202-1, SPAD device 202-2, SPAD device 202-3, SPAD device 202-4, .. . , SPAD device 202-N). More than two SPAD devices, more than ten SPADdevices, more than one hundred SPAD devices, more than one thousand SPADdevices, etc. may be included in a given silicon photomultiplier.

Herein, each SPAD device may be referred to as a SPAD pixel 202.Although not shown explicitly in FIG. 2, readout circuitry for thesilicon photomultiplier may measure the combined output current from allof SPAD pixels in the silicon photomultiplier. In this way, the dynamicrange of an imaging system including the SPAD pixels may be increased.Each SPAD pixel is not guaranteed to have an avalanche current triggeredwhen an incident photon is received. The SPAD pixels may have anassociated probability of an avalanche current being triggered when anincident photon is received. There is a first probability of an electronbeing created when a photon reaches the diode and then a secondprobability of the electron triggering an avalanche current. The totalprobability of a photon triggering an avalanche current may be referredto as the SPAD's photon-detection efficiency (PDE). Grouping multipleSPAD pixels together in the silicon photomultiplier therefore allows fora more accurate measurement of the incoming incident light. For example,if a single SPAD pixel has a PDE of 50% and receives one photon during atime period, there is a 50% chance the photon will not be detected. Withthe silicon photomultiplier 220 of FIG. 2, chances are that two of thefour SPAD pixels will detect the photon, thus improving the providedimage data for the time period.

The example of a plurality of SPAD pixels having a common output in asilicon photomultiplier is merely illustrative. In the case of animaging system including a silicon photomultiplier having a commonoutput for all of the SPAD pixels, the imaging system may not have anyresolution in imaging a scene (e.g., the silicon photomultiplier canjust detect photon flux at a single point). It may be desirable to useSPAD pixels to obtain image data across an array to allow a higherresolution reproduction of the imaged scene. In cases such as these,SPAD pixels in a single imaging system may have per-pixel readoutcapabilities. Alternatively, an array of silicon photomultipliers (eachincluding more than one SPAD pixel) may be included in the imagingsystem. The outputs from each pixel or from each silicon photomultipliermay be used to generate image data for an imaged scene. The array may becapable of independent detection (whether using a single SPAD pixel or aplurality of SPAD pixels in a silicon photomultiplier) in a line array(e.g., an array having a single row and multiple columns or a singlecolumn and multiple rows) or an array having more than ten, more thanone hundred, or more than one thousand rows and/or columns.

While there are a number of possible use cases for SPAD pixels asdiscussed above, the underlying technology used to detect incident lightis the same. All of the aforementioned examples of devices that use SPADpixels may collectively be referred to as SPAD-based semiconductordevices. A silicon photomultiplier with a plurality of SPAD pixelshaving a common output may be referred to as a SPAD-based semiconductordevice. An array of SPAD pixels with per-pixel readout capabilities maybe referred to as a SPAD-based semiconductor device. An array of siliconphotomultipliers with per-silicon-photomultiplier readout capabilitiesmay be referred to as a SPAD-based semiconductor device.

An imaging system 10 with a SPAD-based semiconductor device is shown inFIG. 3. Imaging system 10 may be an electronic device such as a digitalcamera, a computer, a cellular telephone, a medical device, or otherelectronic device. Imaging system 10 may be an imaging system on avehicle (sometimes referred to as vehicular imaging system). Imagingsystem may be used for LIDAR applications.

Imaging system 14 may include one or more SPAD-based semiconductordevices 14 (sometimes referred to as semiconductor devices 14, devices14, SPAD-based image sensors 14, or image sensors 14). One or morelenses 28 may optionally cover each semiconductor device 14. Duringoperation, lenses 28 (sometimes referred to as optics 28) may focuslight onto SPAD-based semiconductor device 14. SPAD-based semiconductordevice 14 may include SPAD pixels that convert the light into digitaldata. The SPAD-based semiconductor device may have any number of SPADpixels (e.g., hundreds, thousands, millions, or more).

The SPAD-based semiconductor device 14 may optionally include additionalcircuitry such as bias circuitry (e.g., source follower load circuits),sample and hold circuitry, correlated double sampling (CDS) circuitry,amplifier circuitry, analog-to-digital (ADC) converter circuitry, dataoutput circuitry, memory (e.g., buffer circuitry), address circuitry,etc.

Image data from SPAD-based semiconductor device 14 may be provided toimage processing circuitry 16. Image processing circuitry 16 may be usedto perform image processing functions such as automatic focusingfunctions, depth sensing, data formatting, adjusting white balance andexposure, implementing video image stabilization, face detection, etc.For example, during automatic focusing operations, image processingcircuitry 16 may process data gathered by the SPAD pixels to determinethe magnitude and direction of lens movement (e.g., movement of lens 28)needed to bring an object of interest into focus. Image processingcircuitry 16 may process data gathered by the SPAD pixels to determine adepth map of the scene.

Imaging system 10 may provide a user with numerous high-level functions.In a computer or advanced cellular telephone, for example, a user may beprovided with the ability to run user applications. To implement thesefunctions, the imaging system may include input-output devices 22 suchas keypads, buttons, input-output ports, joysticks, and displays.Additional storage and processing circuitry such as volatile andnonvolatile memory (e.g., random-access memory, flash memory, harddrives, solid state drives, etc.), microprocessors, microcontrollers,digital signal processors, application specific integrated circuits,and/or other processing circuits may also be included in the imagingsystem.

Input-output devices 22 may include output devices that work incombination with the SPAD-based semiconductor device. For example, alight-emitting component may be included in the imaging system to emitlight (e.g., infrared light or light of any other desired type).Semiconductor device 14 may measure the reflection of the light off ofan object to measure distance to the object in a LIDAR (light detectionand ranging) scheme.

FIG. 4 shows one example for a semiconductor device 14 that includes anarray 120 of SPAD pixels 202 (sometimes referred to herein as imagepixels or pixels) arranged in rows and columns. Array 120 may contain,for example, hundreds or thousands of rows and columns of SPAD pixels202. Each SPAD pixel may be coupled to an analog pulse counter thatgenerates a corresponding pixel voltage based on received photons. EachSPAD pixel may be additionally or instead be coupled to a time-of-flightto voltage converter circuit. In both types of readout circuits,voltages may be stored on pixel capacitors and may later be scanned in arow-by-row fashion. Control circuitry 124 may be coupled to row controlcircuitry 126 and image readout circuitry 128 (sometimes referred to ascolumn control circuitry, readout circuitry, processing circuitry, orcolumn decoder circuitry). Row control circuitry 126 may receive rowaddresses from control circuitry 124 and supply corresponding rowcontrol signals to SPAD pixels 202 over row control paths 130. One ormore conductive lines such as column lines 132 may be coupled to eachcolumn of pixels 202 in array 120. Column lines 132 may be used forreading out image signals from pixels 202 and for supplying bias signals(e.g., bias currents or bias voltages) to pixels 202. If desired, duringpixel readout operations, a pixel row in array 120 may be selected usingrow control circuitry 126 and image signals generated by image pixels202 in that pixel row can be read out along column lines 132.

Image readout circuitry 128 may receive image signals (e.g., analog ordigital signals from the SPAD pixels) over column lines 132. Imagereadout circuitry 128 may include sample-and-hold circuitry for samplingand temporarily storing image signals read out from array 120, amplifiercircuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry,column memory, latch circuitry for selectively enabling or disabling thecolumn circuitry, or other circuitry that is coupled to one or morecolumns of pixels in array 120 for operating pixels 202 and for readingout signals from pixels 122. ADC circuitry in readout circuitry 128 mayconvert analog pixel values received from array 120 into correspondingdigital pixel values (sometimes referred to as digital image data ordigital pixel data). Alternatively, ADC circuitry may be incorporatedinto each SPAD pixel 202. Image readout circuitry 128 may supply digitalpixel data to control and processing circuitry 124 and/or imageprocessing and data formatting circuitry 16 (FIG. 1) over path 125 forpixels in one or more pixel columns.

The example of image sensor 14 having readout circuitry to read outsignals from the SPAD pixels in a row-by-row manner is merelyillustrative. In other embodiments, the readout circuitry in the imagesensor may simply include digital pulse counting circuits coupled toeach SPAD pixel. Any other desired readout circuitry arrangement may beused.

If desired, array 120 may be part of a stacked-die arrangement in whichpixels 202 of array 120 are split between two or more stackedsubstrates. Alternatively, pixels 202 may be formed in a first substrateand some or all of the corresponding control and readout circuitry maybe formed in a second substrate. Each of the pixels 202 in the array 120may be split between the two dies at any desired node within pixel.

It should be understood that instead of having an array of SPAD pixelsas in FIG. 4, SPAD-based semiconductor device 14 may instead have anarray of silicon photomultipliers (each of which includes multiple SPADpixels with a common output).

As shown in FIG. 5, each SPAD pixel 202 in a group 220 of SPAD devices(see FIG. 2) may be covered by a microlens 502 (alternatively, each SPADpixel 202 in the array of SPAD pixels shown in FIG. 4 may be covered bya microlens 502). In particular, each microlens 502 may focus incidentlight on an associated one of SPAD pixels 202. In general, microlenses502 may be formed by any desired method. As an example, microlensmaterial may be applied over SPAD pixels 202 and the reflowed to formmicrolenses 502. However, this merely illustrative. Any desired methodmay be used to form microlenses 502.

Regardless of the method used to form the microlenses, they may be toothin to focus light properly on the photosensitive regions when formedusing conventional manufacturing methods and equipment. In particular,the SiPM devices may have SPAD pixels with pitches that areapproximately between 20 microns and 35 microns wide. To focus lightincident on image sensor onto the array of SPAD pixels, sphericalmicrolenses with thicknesses of approximately 20 microns may berequired. This is thicker than traditional microlenses, which may havethicknesses of approximately 5 microns, and standard equipment maytherefore not be capable of forming microlenses for SiPM devices. Toform microlenses with higher thicknesses, microlens material with ahigher viscosity may be used. However, this poses additional issues, asuniformity, photo patterning, and reflow characteristics may bechallenging when using high-viscosity material. For example, adjacentmicrolenses may merge together, as it may be difficult to maintain theshape and size of the microlenses, such as microlenses 502, duringreflow operations. Therefore, it may be desired to use additionalstructures to control the formation of microlenses.

As shown in FIG. 6, microlenses 602 may be formed on substrate 604.Substrate 604 may be a silicon substrate or may be formed from anydesired material. SPAD pixels, such as SPAD pixels 202, may be formed insubstrate 604, or substrate 604 may overlap SPAD pixels 202. In anycase, microlenses 602 may overlap SPAD pixels (or other desired type ofpixels) and direct light into the pixels.

As shown in FIG. 6, SPAD pixels 202 may optionally be formed insubstrate 604. Each SPAD pixel 202 may have an active area 608 and aninactive area 610. Inactive area 610 may contain circuitry and space theSPAD pixels apart for more accurate photon detection (e.g., the spacebetween pixels may reduce crosstalk between the pixels). Active area 608may be sensitive to photons of incoming light. As a result, microlenses602 may be formed over active areas 608, and containment grid 606 may beformed over inactive areas 610. If desired, however, portions ofcontainment grid 606 may extend at least partially into active area 608.As an example, a tapered containment grid portion, such as grid portion602-1 may overlap only inactive region 610 at substrate 604, but mayflare up to additionally overlap a portion of active region 608.However, this is merely illustrative. Containment grid 606 may beconfined to inactive region 610 or may extend partially into activeregion 608.

Microlenses 602 may be separated by containment grid 606. Containmentgrid 606 may help contain microlenses 602 within openings within thecontainment grid during reflow operations. In other words, thecontainment grid may prevent adjacent microlenses from merging. As shownin FIG. 6, containment grid portions 606-1 and 606-2 may help containmicrolens 602-1, containment grid portions 606-2 and 606-3 may helpcontain microlens 602-2, and containment grid portions 606-3 and 606-4may help contain microlens 602-3. The containment grid portions may havetapered shapes, as shown by containment grid portions 606-1, 606-2, and606-3, may have flat walls, as shown by containment grid portion 606-4,or may have any other desired shape. In some embodiments, containmentgrid portions may have tapered shapes to help contain microlens materialduring the formation of microlenses 602. In general, containment gridportions 606 may all have substantially the same shape over the array ofSPAD pixels 202, or may vary in shape across the array of pixels.

In some embodiments, the containment grid 606 may include material witha lower index of refraction than the microlens material (e.g., thematerial used to form microlenses 602). This may allow containment gridportions 606 to absorb high-angle light during operation of the SPADpixels, thereby improving the accuracy of detection by the underlyingSPAD pixels. Containment grid 606 may be formed from black material,metal, metal oxide, or dielectric materials, as examples. In general,any desired material may be used to form containment grid 606. In somecases, black or metal material may be used to absorb off-angle light. Byblocking off-angle light, containment grid 606 may reduce crosstalkbetween adjacent SPAD pixels 202. In particular, crosstalk in SiPM/SPADdevices often occurs due to the emission of light of one pixel moving toan adjacent pixel and being absorbed. This is known as secondary photongeneration crosstalk. By forming containment grid 606 from blackmaterial, metal material, or other absorptive material, the containmentgrid may absorb the generated photons and reduce crosstalk detected byneighboring pixels. In some embodiments, containment grid 606 may beextended at least partially into substrate 604 to provide absorption forlight at higher angles.

Although containment grid portions 606 are shown as being much thinnerthan microlenses 602, this is merely illustrative. In general,containment grid portions 606 may extend to any desired height fromsubstrate 604. For example, the containment grid may be made thicker tohelp converge normal incident light within silicon substrate 604.Alternatively, the containment grid may be made thinner to better focusoff-angle light. In some embodiments, the microlenses may be at leasttwo times thicker than the containment grid, at least three timesthicker than the containment grid, at least ten times thicker than thecontainment grid, less than 15 times thicker than the containment grid,or at least five times thicker than the containment grid, as examples.Microlenses 602 may have thicknesses of less than 25 microns, less than20 microns, less than 10 microns, less than 5 microns, greater than 3microns, or less than 4 microns. However, the thickness of containmentgrid portions 606 and microlenses 602 may be adjusted as desired.

Moreover, although microlenses 602 are shown in FIG. 6 as being entirelybetween containment grid portions 606, this is merely illustrative. Insome embodiments, microlenses 602 may at least partially overlapcontainment grid portions 606. For example, edge portions of microlenses602 may overlap containment grid portions 606. This arrangement mayallow microlenses 602 to have an increased standoff height relative tosubstrate 604, which in turn may improve the focusing ability ofmicrolenses 602.

Although the example of FIG. 6 shows the microlenses applied directlyover SPAD pixels 602 (e.g., a backside illuminated arrangement), acontainment grid can also be used when forming microlenses overfrontside illuminated image sensors, as well.

Microlenses 602 and containment grid 606 may be formed using any desiredmethod. However, as previously discussed, it may be desirable to formmicrolenses 602 using a reflow process, so that microlens material maybe applied across an array of SPAD pixels and then reflowed to shape thelenses. An illustrative process by which containment grid 606 andmicrolenses 602 may be formed is shown in FIG. 7.

As shown in FIG. 7, containment grid material 702 may be deposited onsubstrate 604. As discussed, substrate 604 may be any desired material,such as silicon. Containment grid material 702 may be metal, metaloxide, black material, or any other desired material. In someembodiments, containment grid material 702 may be configured to absorblight.

After containment grid material 702 has been deposited, the process flowmay proceed along arrow 704, and the containment grid may be patternedto form openings in which microlenses will be formed. Any desired methodmay be used to pattern the containment grid, such as photolithography oretching. As shown in FIG. 7, containment grid portions 706-1 and 706-2may at least partially surround opening 708-1, and containment gridportions 706-2 and 706-3 may at least partially surround opening 708-2.These openings may extend in two dimensions across the array of SPADpixels 14.

Although containment grid portions 706 are shown as having flat walls,some or all of containment grid portions 706 may have tapered shapes,like containment grid portions 606-1, 606-2, and 606-3 of FIG. 6. Thismay help contain microlens material during formation of the microlenses.However, this shape is merely illustrative. In general, containment gridportions 706 may have any desired shapes.

The process may then proceed along arrow 710, and microlens material 712may be deposited over containment grid 706 and substrate 604. Microlensmaterial 712 may be formed from acrylic, silicon, any other desiredmaterial, or any desired combinations of materials. If desired,microlens material 712 may have a higher index of refraction thancontainment grid material 702. In this way, high-angle light may beredirected by microlens material 712 and be detected by the underlyingSPAD pixels. However, this is merely illustrative. In general, anydesired material may be used for microlens material 712.

After depositing microlens material 712, the process may proceed alongarrow 714, and microlens material 712 may be patterned to form an arrayof patterned portions 716, which includes patterned portions 716-1 and716-2. Microlens material 712 may be patterned using any desiredtechnique, such as photolithography or etching. Although gaps are shownbetween patterned portions 716 and containment grid 706, this is merelyillustrative. In some examples, it may be desirable to have less of agap or no gap between patterned portions 716 and containment grid 706,as doing so may allow microlenses to overlap containment grid 706 andcreate additional standoff height from substrate 604.

The process may then proceed along arrow 718, and the patterned portions716 may be reflowed to form an array of microlenses 720. All of thepatterned portions 716 may be reflowed simultaneously, or some of thepatterned portions 716 may be reflowed before other patterned portions716. Patterned portions 716 may be reflowed in any desired manner toform microlenses 720. Microlenses 720 may have a spherical shape or anyother desired shape. Moreover, microlens 720-1 may have the same shapeas microlens 720-2 or the microlenses may have different shapes. Ingeneral, the reflow processes may be adjusted to create any desiredshapes for microlenses 720 across the array of microlenses.

As shown, containment grid portions 706-1, 706-2, and 706-3 may helpprevent microlenses 720-1 and 720-2 from merging during reflowoperations. In particular, the containment grid portions may act asbarriers to contain the microlens material as it is being reflowed.After formation of the microlenses, containment grid 706 may absorboff-axis secondary photons generated within the avalanche region of theSPAD, which could otherwise reflect back into adjacent SPADs, creatingcross-talk. For example, containment grid 706 may be formed from blackmaterial, metal, or other light absorptive material to help preventcross talk between SPAD pixels.

Although the method of FIG. 7 allows for the formation of microlenseswithin a containment grid, it may be desirable to use phobic material toprevent neighboring microlenses from merging above the containment grid.A process diagram illustrating the use of phobic material is shown inFIG. 8.

As shown in FIG. 8, containment grid material 802 may be deposited onsubstrate 604. As discussed, substrate 604 may be any desired material,such as silicon. Containment grid material 802 may be metal, metaloxide, black material, or any other desired material. In particular,containment grid material 802 may be phyllic with respect to microlensmaterial. Therefore, the containment grid material may be silicon,oxides, or any other material that promotes the attachment of microlensmaterial. In some embodiments, containment grid material 802 may also beconfigured to absorb light.

Phobic material 803 may be deposited on containment grid material 802.In particular, phobic material 803 may adhere poorly to microlensmaterial. In general, phobic material 803 may be any desired material,such as a fluoropolymer.

After containment grid material 802 and phobic material 803 have beendeposited, the process flow may proceed along arrow 804, and thecontainment grid and phobic material may be patterned to form openingsin which microlenses will be formed. Any desired method may be used topattern the containment grid and phobic material, such asphotolithography or etching. As shown in FIG. 8, containment gridportions 806-1 and 806-2 and phobic portions 807-1 and 807-2 may atleast partially surround opening 808-1, and containment grid portions806-2 and 806-3 and phobic portions 807-2 and 807-3 may at leastpartially surround opening 808-2. These openings may extend in twodimensions across the array of SPAD pixels 14.

Although containment grid portions 806 are shown as having taperedshapes, some or all of containment grid portions 806 may have flatwalls, like containment grid portion 606-4 of FIG. 6. However, thisshape is merely illustrative. In general, containment grid portions 806may have any desired shapes.

The process may then proceed along arrow 810, and microlens material 812may be deposited over containment grid 806, phobic material 807, andsubstrate 604. Microlens material 812 may be formed from acrylic,silicon, any other desired material, or any desired combinations ofmaterials. If desired, microlens material 812 may have a higher index ofrefraction than containment grid material 802. In this way, high-anglelight may be redirected by microlens material 812 and be detected by theunderlying SPAD pixels. However, this is merely illustrative. Ingeneral, any desired material may be used for microlens material 812.

After depositing microlens material 812, the process may proceed alongarrow 814, and microlens material 812 may be patterned to form an arrayof patterned portions 816, which includes patterned portions 816-1 and816-2. Microlens material 812 may be patterned using any desiredtechnique, such as photolithography or etching. Although gaps are shownbetween patterned portions 816 and containment grid 806, this is merelyillustrative. In some examples, it may be desirable to have less of agap or no gap between patterned portions 816 and containment grid 806,as doing so may allow microlenses to overlap containment grid 806 andcreate additional standoff height from substrate 604.

The process may then proceed along arrow 818, and the patterned portions816 may be reflowed to form an array of microlenses 820. All of thepatterned portions 816 may be reflowed simultaneously, or some of thepatterned portions 816 may be reflowed before other patterned portions816. Patterned portions 816 may be reflowed in any desired manner toform microlenses 820. Microlenses 820 may have a spherical shape or anyother desired shape. Moreover, microlens 820-1 may have the same shapeas microlens 820-2 or they may have different shapes. In general, thereflow processes may be adjusted to create any desired shapes formicrolenses 820 across the array of microlenses.

As shown, containment grid portions 806-1, 806-2, and 806-3 may helpprevent microlenses 820-1 and 820-2 from merging during reflowoperations. In particular, the containment grid portions may act asbarriers to contain the microlens material as it is being reflowed.Additionally, phobic portions 822 may help prevent adjacent microlenses820 from merging because the phobic material may be resistant to themicrolens material. After formation of the microlenses, containment grid806 may absorb off-axis light secondary photons generated within theavalanche region of the SPAD, which could otherwise reflect back intoadjacent SPADs, creating cross-talk. For example, containment grid 806may be formed from black material, metal, or other light absorptivematerial to help prevent cross talk between SPAD pixels.

In some cases, it may be desirable to use containment grid material thatis phobic to microlens material. A process diagram illustrating the useof phobic containment grid material is shown in FIG. 9.

As shown in FIG. 9, containment grid material 902 may be deposited onsubstrate 604. As discussed, substrate 604 may be any desired material,such as silicon. Containment grid material 902 may be metal, metaloxide, black material, or any other desired material. In particular,containment grid material 902 may be phobic with respect to microlensmaterial. Therefore, the containment grid material may be metal, oxides,or any other material that resists the attachment of microlens material.In some embodiments, containment grid material 902 may also beconfigured to absorb light.

After containment grid material 902 has been deposited, the process flowmay proceed along arrow 904, and the containment grid may be patternedto form openings in which microlenses will be formed. Any desired methodmay be used to pattern the containment grid and phobic material, such asphotolithography or etching. Additionally, material that is phyllic tomicrolens material may be applied over containment grid portions 906.Phyllic material 907 may cover the top and sides of each of containmentgrid portions 906-1, 906-2, and 906-3. Alternatively, phyllic material907 may cover only part of the containment grid portions. Phyllicmaterial 907 may be acrylic, silicon, resin, oxides, or any otherdesired material that promotes adhesion to microlens material. Ingeneral, phyllic material 907 may cover any desired portion of theunderlying containment portions and may be formed from any desiredmaterial.

As shown in FIG. 9, containment grid portions 906-1 and 906-2 andphyllic portions 907-1 and 907-2 may at least partially surround opening908-1, and containment grid portions 906-2 and 906-3 and phobic portions907-2 and 907-3 may at least partially surround opening 908-2. Theseopenings may extend in two dimensions across the array of SPAD pixels14.

Although containment grid portions 906 are shown as having taperedshapes, some or all of containment grid portions 906 may have flatwalls, like containment grid portion 606-4 of FIG. 6. However, thisshape is merely illustrative. In general, containment grid portions 906may have any desired shapes.

The process may then proceed along arrow 910, and top portions of thephyllic material may be removed from each of the containment gridportions. As shown, this may expose upper surface 909 of eachcontainment grid portion, and leave phyllic portions on the edgesurfaces (as illustrated by phyllic portions 907-3A and 907-3B). Thephyllic material may be removed using any desired process, such asetching.

Although FIG. 9 illustrates removing only a top portion of the phyllicmaterial over each containment grid portion, this is merelyillustrative. Any portion or portions of the phyllic material may beremoved, as desired.

After removing the desired portions of the phyllic material, the processmay proceed along arrow 911, and microlens material 912 may be depositedover containment grid 906, phyllic portions 907, and substrate 604.Microlens material 912 may be formed from acrylic, silicon, any otherdesired material, or any desired combinations of materials. If desired,microlens material 912 may have a higher index of refraction thancontainment grid material 902. In this way, high-angle light may beredirected by microlens material 812 and be detected by the underlyingSPAD pixels. In some embodiments, microlens material 912 may be the samematerial as the material used to form phyllic portions 907, therebypromoting adhesion between the microlens material and the phyllicportions. However, this is merely illustrative. In general, any desiredmaterial may be used for microlens material 912.

After depositing microlens material 912, the process may proceed alongarrow 914, and microlens material 912 may be patterned to form an arrayof patterned portions 916, which includes patterned portions 916-1 and916-2. Microlens material 912 may be patterned using any desiredtechnique, such as photolithography or etching. Although gaps are shownbetween patterned portions 916 and containment grid 906, this is merelyillustrative. In some examples, it may be desirable to have less of agap or no gap between patterned portions 916 and containment grid 906,as doing so may allow microlenses to overlap containment grid 906 andcreate additional standoff height from substrate 604.

The process may then proceed along arrow 918, and the patterned portions916 may be reflowed to form an array of microlenses 920. All of thepatterned portions 916 may be reflowed simultaneously, or some of thepatterned portions 916 may be reflowed before other patterned portions916. Patterned portions 916 may be reflowed in any desired manner toform microlenses 920. Microlenses 920 may have a spherical shape or anyother desired shape. Moreover, microlens 920-1 may have the same shapeas microlens 920-2 or they may have different shapes. In general, thereflow processes may be adjusted to create any desired shapes formicrolenses 920 across the array of microlenses.

As shown, containment grid portions 906-1, 906-2, and 906-3 may helpprevent microlenses 920-1 and 920-2 from merging during reflowoperations. In particular, the containment grid portions may act asbarriers to contain the microlens material as it is being reflowed. Dueto the presence of phyllic portions 907 along the sides of thecontainment grid portions, the microlens material may be attracted tothe phyllic portions during reflow operations. Additionally, since thephyllic material was removed from the top surfaces of the containmentgrid portions and the containment grid portions are formed from phobicmaterial, the microlens material may not adhere well to these surfaces.This may help prevent merging between adjacent microlenses. Afterformation of the microlenses, containment grid 906 may absorb off-axissecondary photons generated within the avalanche region of the SPAD,which could otherwise reflect back into adjacent SPADs, creatingcross-talk. For example, containment grid 906 may be formed from blackmaterial, metal, or other light absorptive material to help preventcross talk between SPAD pixels.

In any of the aforementioned embodiments, it should be understood that asilicon photomultiplier (with multiple SPAD pixels having a commonoutput) may be used in place of a single SPAD pixel. Each SPAD pixel inthe silicon multiplier may be covered by a microlens, or multiple SPADpixels within the silicon multiplier may be covered by a singlemicrolens, if desired.

Although each of the aforementioned embodiments have been described asapplying a microlens over SPAD pixels, the microlenses may be formedover any desired pixel type. For example, the foregoing microlenses maybe applied over pixels in conventional CMOS imagers.

In accordance with an embodiment, a semiconductor device may include aplurality of single-photon avalanche diode pixels. Each of thesingle-photon avalanche diode pixels may have an active region and aninactive region. The semiconductor device may also include a pluralityof microlenses, each of which covers the active region of a respectiveone of the single-photon avalanche diode pixels, and a containment gridthat covers the inactive regions of the single-photon avalanche diodepixels. Portions of the containment grid may be interposed betweenadjacent microlenses of the plurality of microlenses.

In accordance with various embodiments, the portions of the containmentgrid may each have a tapered shape between the microlenses.

In accordance with various embodiments, the portions of the containmentgrid may each have flat sidewalls between the microlenses.

In accordance with various embodiments, the containment grid may includecontainment grid material with a first index of refraction, themicrolenses may include microlens material with a second index ofrefraction, and the second index of refraction may be higher than thefirst index of refraction.

In accordance with various embodiments, the containment grid may includea material selected from the group consisting of: metal material, metaloxide material, silicon material and black material.

In accordance with various embodiments, each of the containment gridportions may have a top surface, and the semiconductor device mayfurther include material that is phobic to the microlens material on atleast some of the top surfaces.

In accordance with various embodiments, the containment grid may includematerial that is phobic to the microlens material.

In accordance with various embodiments, the semiconductor device mayfurther include material that is phyllic to the microlens materialinterposed between at least some of the containment grid portions andthe microlenses.

In accordance with various embodiments, each of the microlenses may havea first height, the containment grid may have a second height, and thefirst height may be greater than the second height.

In accordance with various embodiments, the first height may be at leastten times greater than the second height.

In accordance with an embodiment, a method of forming microlenses over aplurality of single-photon avalanche diodes may include depositingcontainment grid material on a semiconductor substrate, patterning thecontainment grid material to form an array of openings, depositingmicrolens material over the containment grid material and thesemiconductor substrate, and patterning and reflowing the microlensmaterial to form microlenses in the openings of the containment gridmaterial.

In accordance with various embodiments, the method may further includedepositing phobic material over the containment grid material, andpatterning the containment grid material to form an array of openingsmay include patterning the containment grid material and the phobicmaterial.

In accordance with various embodiments, the containment grid may bephobic to the microlens material, and the method may further includebefore depositing the microlens material, depositing phyllic material onthe patterned containment grid material, and etching a surface of thephyllic material to expose a portion of the containment grid material.

In accordance with various embodiments, depositing the containment gridmaterial may include depositing the containment grid material to a firstheight from semiconductor substrate, and depositing the microlensmaterial may include depositing the microlens material to a secondheight from the substrate that is at least ten times greater than thefirst height.

In accordance with various embodiments, patterning the containment gridmaterial may include forming containment grid portions having a shapeselected from the group consisting of: a tapered shape and a flat-walledshape.

In accordance with an embodiment, a semiconductor device may include asingle-photon avalanche diode pixel having an active region and aninactive region, a containment grid having portions that cover theinactive region and having an opening that overlap the active region,and a microlens in the opening of the containment grid that overlaps theactive region.

In accordance with various embodiments, the single-photon avalanchediode pixel may be a pixel in an array of single-photon avalanche diodepixels, and the containment grid may include containment grid materialthat absorbs stray light and prevents cross talk between adjacent pixelsin the array of pixels.

In accordance with various embodiments, the containment grid materialmay be selected from the group of material consisting of: black materialand metal oxide material.

In accordance with various embodiments, the microlens may includemicrolens material, and the containment grid material may be phobic tothe microlens. The semiconductor device may further include additionalmaterial that is phyllic to the microlens material interposed betweenthe containment grid material and the microlens material.

In accordance with various embodiments, the containment grid materialmay have a first index of refraction and the microlens may be formedfrom microlens material that has a second index of refraction that isgreater than the first index of refraction.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the art. Theforegoing embodiments may be implemented individually or in anycombination.

What is claimed is:
 1. A semiconductor device comprising: a plurality ofsingle-photon avalanche diode pixels, wherein each of the single-photonavalanche diode pixels has an active region and an inactive region; aplurality of microlenses, wherein each of the microlenses covers theactive region of a respective one of the single-photon avalanche diodepixels; and a containment grid that covers the inactive regions of thesingle-photon avalanche diode pixels, wherein portions of thecontainment grid are interposed between adjacent microlenses of theplurality of microlenses.
 2. The semiconductor device defined in claim 1wherein the portions of the containment grid each have a tapered shapebetween the microlenses.
 3. The semiconductor device defined in claim 1wherein the portions of the containment grid each have flat sidewallsbetween the microlenses.
 4. The semiconductor device defined in claim 1wherein the containment grid comprises containment grid material with afirst index of refraction, the microlenses comprises microlens materialwith a second index of refraction, and the second index of refraction ishigher than the first index of refraction.
 5. The semiconductor devicedefined in claim 1 wherein the containment grid comprises a materialselected from the group consisting of: metal material, metal oxidematerial, silicon material, and black material.
 6. The semiconductordevice defined in claim 1 wherein each of the containment grid portionshas a top surface, the semiconductor device further comprising materialthat is phobic to the microlens material on at least some of the topsurfaces.
 7. The semiconductor device defined in claim 1 wherein thecontainment grid comprises material that is phobic to the microlensmaterial.
 8. The semiconductor device defined in claim 7 furthercomprising: material that is phyllic to the microlens materialinterposed between at least some of the containment grid portions andthe microlenses.
 9. The semiconductor device defined in claim 1 whereinthe each of the microlenses has a first height, the containment grid hasa second height, and the first height is greater than the second height.10. The semiconductor device defined in claim 9 wherein the first heightis at least ten times greater than the second height.
 11. A method offorming microlenses over a plurality of single-photon avalanche diodes,the method comprising: depositing containment grid material on asemiconductor substrate; patterning the containment grid material toform an array of openings; depositing microlens material over thecontainment grid material and the semiconductor substrate; andpatterning and reflowing the microlens material to form microlenses inthe openings of the containment grid material.
 12. The method defined inclaim 11 further comprising: depositing phobic material over thecontainment grid material, wherein patterning the containment gridmaterial to form an array of openings comprises patterning thecontainment grid material and the phobic material.
 13. The methoddefined in claim 11 wherein the containment grid material is phobic tothe microlens material, the method further comprising: before depositingthe microlens material, depositing phyllic material on the patternedcontainment grid material; and etching a surface of the phyllic materialto expose a portion of the containment grid material.
 14. The methoddefined in claim 13 wherein depositing the containment grid materialcomprises depositing the containment grid material to a first heightfrom semiconductor substrate, and wherein depositing the microlensmaterial comprises depositing the microlens material to a second heightfrom the substrate that is at least ten times greater than the firstheight.
 15. The method defined in claim 11 wherein patterning thecontainment grid material comprises forming containment grid portionshaving a shape selected from the group consisting of: a tapered shapeand a flat-walled shape.
 16. A semiconductor device comprising: asingle-photon avalanche diode pixel having an active region and aninactive region; a containment grid having portions that cover theinactive region, wherein the containment grid has an opening thatoverlap the active region; and a microlens in the opening of thecontainment grid that overlaps the active region.
 17. The semiconductordevice defined in claim 16 wherein the single-photon avalanche diodepixel is a pixel in an array of single-photon avalanche diode pixels,and wherein the containment grid comprises containment grid materialthat is configured to absorb stray light and prevent cross talk betweenadjacent pixels in the array of pixels.
 18. The semiconductor devicedefined in claim 17 wherein the containment grid material is selectedfrom the group of material consisting of: black material and metal oxidematerial.
 19. The semiconductor device defined in claim 18 wherein themicrolens comprises microlens material and wherein the containment gridmaterial is phobic to microlens material, the semiconductor devicefurther comprising: additional material that is phyllic to the microlensmaterial interposed between the containment grid material and themicrolens material.
 20. The semiconductor device defined in claim 17wherein the containment grid material has a first index of refractionand wherein the microlens is formed from microlens material that has asecond index of refraction that is greater than the first index ofrefraction.